Method for force, displacement, and rate control of shaped memory material implants

ABSTRACT

A method for force, displacement, and rate control of shape memory material implant. The rate of implant shape change as well as the force exerted on the surrounding tissue of the implant can be controlled by the surgeon and the extent of movement is controlled in circumstances where the bone element is free to move. The invention allows for the first time the fine control of force when fixating osteoporotic bone and rate of bone transport when working near the spinal cord. This heating profile of the implant provides the surgeon to control the extent of microstructure phase transformation so that the rate, force or extent of tissue movement can be controlled individually or together.

FIELD OF THE INVENTION

The present invention relates to instrumentation and a method ofcontrolling in vivo shape changes in devices formed from memorymaterials that change shape when heated. In particular, the presentinvention relates to surgical instrumentation, used to control the rateof shape change and forces imparted to the surrounding tissue by memorymaterial implants during medical use.

BACKGROUND-DISCUSSION OF THE PRIOR ART

Shape memory alloys such as nitinol have been well known since theirdevelopment by Buehler and Wiley (U.S. Pat. No. 3,174,851) in 1965.Other metals, such as AuCd, FePt₃, beta Brass and InTI, exhibit shapememory behavior. These materials have the property of changing shape inresponse to a change in material temperature. This shape changepotential is imparted into the memory metal device through a series ofheat treatments.

The transition temperature range is imparted to the material throughvarying mixtures of intermetallic compounds such as nickel-titanium andheat treatment. The heat treatment methods for the material generallyconsist of at a minimum high temperature setting of the desired finalshape of a device followed by a low temperature straining of the deviceto a second shape. Then when the device in the second shape and broughtto the transition temperature the device returns to the preprogrammedfinal shape. The shape change occurs due to the transition of thematerial from a martensitic to austenitic phase microstructure. Theseheat-initiated changes cause gross changes in the shape of the implantformed from the memory metal.

Shape memory alloys have been used for a wide range of industrial andmedical applications. Medical applications include but are not limitedto: catheter, intrauterine contraceptive device, gastrointestinalcompression-dip, blood vessel filter, coronary artery stent, skinstaple, bone staple, and bone plate.

In the prior art memory metal implants have been caused to change shapeby heating by their environment, applied current or directed energy.Implants and surgical instruments that change shape in the environmentof human body temperature have been described by Jervis and include butare not limited to laprascopic instruments, needle and suturemanipulation devices, and expansion and shrinkable coil stents andclosures. Implants, that change shape using the Joule effect throughresistive heating, have been described by Krumme, Alfidi and Flot. Theprior art associated with resistive heating of memory alloys have notrecognized the need for the control of the rate of shape change andmagnitude of forces applied by the implant to the surrounding tissue.Furthermore, the prior art describes the full transition of thematerial, from martensitic to austinetic microstructure through thedelivery of either: 1) a predetermined amount of heat energy to aspecific implant (Flot), 2) opening to an initial configuration (Alfidi)or 3) until the shape change breaks contact with current carryingelectrodes (Krumme).

Only Jervis recognized the need for force and shape control of amusculoskeletal implant but controlled these through a mechanicalactuator that resisted the heat induced shape change. This approach wasrequired due to the transition temperature of the metal fully convertingto the austinetic phase at body temperature. In this case, there was noforce control of the implant due to the body temperature transitionpoint of the metal, which resulted in the implant applying the maximumpotential force to the surrounding tissue.

The prior art is at a significant disadvantage to the subject inventionin the field of orthopaedics due to the lack of a method and system tocontrol the rate and maximum shape change force exerted on thesurrounding tissue. The clear advantages of the subject invention willbe seen in the following review of the prior art.

Methods of heating memory alloy medical devices to change their shapeinclude: conductive heat transfer (Alfidi [U.S. Pat. No. 3,868,956] andKrumme [U.S. Pat. Nos. 4,550,870 and 4,485,816]), electromagnetic energyheating, and resistive heating using the joule effect (Alfidi [U.S. Pat.No. 3,868,956], Krumme [U.S. Pat. No. 4,485,816] and Flot [U.S. Pat. No.6,268,589 B1 and U.S. Pat. No. 6,323,461 B2]). One source of conductiveheat energy is the ambient temperature of the human body (Jervis: U.S.Pat. No 4,665,906, U.S. Pat. No. 5,067,957, U.S. Pat. No. 5,190,546, andU.S. Pat. No. 5,597,378).

Resistive heating has been found to be a convenient method for medicaldevices. Resistive heating devices have used both AC current (Flot [U.S.Pat. No. 6,268,589 B1]) and DC current (Alfidi [U.S. Pat. No. 3,868,956]and Krumme [U.S. Pat. No. 4,485,816]) to change the implant shape. Thesesystems. control the heating current so as to limit the maximumtemperature (Flot [U.S. Pat. No. 6,268,589 B1]) and (Alfidi [U.S. Pat.No. 3,868,956]) or extent of shape change of the implant (Krumme [U.S.Pat. No. 4,485,816] and Alfidi [U.S. Pat. No. 3,868,956]). Though thesemethods and devices control thermal injury to tissue and extent of shapechange they are significantly limited in musculoskeletal applications.

Krumme (U.S. Pat. No. 4,485,816; col 6, In 37-44) controls the maximumtemperature and extent of shape change by causing contact between theimplant and electrode to break as a result of the shape change. Thissimultaneous secession of heat or electrical energy flow limits heatingof the implant to a level that makes it suitable for use in implantapplications. This implant heating strategy results in a predetermineddegree of shape change but no control of its force or rate of shapechange. This strategy significantly limits implant design because inmany musculoskeletal uses solid stable bone structures may not allow theimplant to change shape only to provide compressive forces. Thus in thisapplication the shape change would not break contact between the implantand electrode and stop -heat energy delivered. Thus the heating deviceof Krumme can not control either rate of shape change or force exertedon the surrounding tissue.

Alfidi [U.S. Pat. No. 3,868,956, col 7, In 20-33] provided time andvoltage controls to limit the energy applied to an implant so as tocontrol the extent of its shape change at temperatures compatible withthe enclosure of the heating element and the biologic environment inwhich the implant is used. Alfidi could monitor the actual current flowover a fixed preset time. Alfidi heated quickly to avoid thermal damage[U.S. Pat. No. 3,868,956, col 3, In 41-43] and expand the “wireappliance to a desired degree”. [U.S. Pat. No. 3,868,956, col 3, In10-15] where desired degree was consistently referenced as “it assumes .. . a configuration . . . which . . . is . . . substantially similar tosaid initial configuration.” [U.S. Pat. No. 3,868,956, col 8, In 61-63].Where the initial configuration is the first shape referenced above thatis formed during the initial high-temperature heat treatment. Thus theheating device of Alfidi can not control force exerted on thesurrounding tissue.

Flot [U.S. Pat. No. 6,268,589 B1 col 1, In 44-47] provided voltagecontrol but removed control of the energy delivery time described byAlfidi [U.S. Pat. No. 3,868,956] from the surgeon to lessen thepotential for overheating the implant and causing tissue injury. Flotmatched an implant size to a specific voltage setting for a fixed periodof time through the use of resistor (R17) on the circuit board [U.S.Pat. No. 6,268,589 B1 col 3, In 21-23]. This matching of implant tovoltage required to provide the complete martensitic to austenitic of animplant is proposed by Flot to be matched to an implant mass so that itdoes not reach a temperature sufficiently high so as to cause thermalnecrosis to surrounding tissue. Flot's approach is limiting in thatwithout control of time the range of implants mass that can be effectedwith this invention is, limited to 0.8 grams to 2.8 grams [U.S. Pat. No.6,268,589 B1 col 3, In 3-7 and col 4, In 37-39]. This occurs due to theimplant's heating profile being dependent only on applied voltagemagnitude and the impedance and mass of the implant. Without usercontrol of both the time and applied voltage the total heating energy toan implant is limited. Thus the implant sizes that can be heated throughtheir transition temperature is limited. The fixed relationship betweenimplant size and control settings presented by Rot teaches against thecontrol of forces exerted by the shape memory alloy implant on thesurrounding tissue. Furthermore the inability to control heat energydelivery time teaches away from controlling the rate ofmemory-metal-implant shape changes so as to protect vital structures.

Jervis, [U.S. Pat. No. 4,665,906 example II and IV] whose medical shapememory allow implants have a transition temperature substantially atbody temperature is the only author of the prior art that realizes theimportance of controlling the rate and force applied by the shapechanging memory alloy implant. Due to the full transition of martensiticto austenitic microstructure occurring in the implant at bodytemperature, Jervis controls the shape change with a “mechanicalrestraint . . . achieving. excellent force and time control, andpermitting the surgeon to make adjustments as desired.” The advantage ofnot requiring a separate instrument to control the closure of theimplant through applied heat energy is greatly overcome by the need fora “mechanical restraint” instrument. This instrument limits the use ofthe shape memory implant. In medicine a different instrument would beneeded for each implant design. Furthermore, many uses would not berealized due to the bulk and functional requirements of the instrumentneeded to control the dosing force and rate of the implant. Finally,once placed and the mechanical restraint removed the implant fullyconverts to the austinetic microstructure and there is no longer anyforce control. Thus the invention of Jervis has significantdisadvantages compared with the subject invention.

The prior art consistently teaches an instrument or techniques totransform the implant to a single final state. The art describes high.temperature setting of an initial shape, low temperature deformation toa second shape state, and heating, of the implant to return the implantto its initial shape. The prior art does not present, .as the subjectinvention describes, a device or method to control the martensitic toaustinetic transformation so that a plurality of fixation forces andrates can be achieved from each of a plurality of implant designs.Jervis's implant reaches the state associated with body temperatureheating. Krumme reaches the state associated with shape change breakingcontact with the current source. Alfidi reaches a configuration that issubstantially similar to said initial configuration. Flot provides“predetermined quantities of heat, each corresponding to a given size ofclamp” but does not provide a plurality of heat energies to a singlestyle damp to control the force applied to bone or its rate of closure.

These limitations of the prior art have caused memory metals to belimited in use in orthopaedics. Memory alloy implants have found use assimple two and four leg staples but have not reached the potential ofimplants that can be manipulated with precise control to change theirshape and move bony structures. The lack of surgeon control of forcesapplied to bone is of significant concern in osteoporotic bone and thuswith implants and heat energy sources described in the prior art.Furthermore the quick shape changing movement of implant and bonystructures described under the prior art could pinch and injury thespinal cord. This inability to adjust the implants shape-changingresponse within the martensitic to austenitic transformation temperaturerange to control its force and closure rate has discouraged the clinicaluse of these systems. The subject invention presents an innovativesolution to these clinical issues

Objects and Advantages

The subject invention for musculoskeletal implant applicationsrecognizes that closing force and rate of shape change of the implantare critical factors in the implants' success due to the wide range ofbone strength, anatomical variation and medical need.

In osteoporotic bone an implant that doses with too much force maycreate fracture. In displaced fractures of healthy bone, implant forcesexerted by its shape change may be too low to pull the fracture lineclosed. If the forces are just right, bone is stimulated to gain massand strengthen. This tendency of bone to adapt to the loads applied toit is described by Wolff's Law. Implants that allow the surgeon tocontrol these forces provide unique clinical benefit in stabilizing bonefractures and applying residual forces that take advantage of Wolff sLaw and advance bone healing. The lack of control, of the peak andresidual forces by the inventions of the prior art, has significantlyimpeded the adoption of musculoskeletal implants that changed shape.

A secondary but significant element of this invention is the ability forthe surgeon to control the rate of shape change of the memory metal. Inthe fixation of bone, memory metals when heated move bone to dosefracture lines or joints intended to be fused. When working in the spinethe movement of vertebra to bring them into apposition to facilitatefusion should be done slowly so as to not cause impingement of thespinal cord. When heating orthopaedic memory metal implants that changethe relative position or angle of bone, it must be done slowly so as toachieve the proper position. The novel feature of controlling the rateof movement of members of bone fixation implants provides the surgeonnew treatment modalities and opens new design possibilities heretoforenot available.

Implants may have multiple members that change shape. Some of thesemembers may be controlled individually and others may need to becontrolled together. These multi-member heating strategies may beaccomplished by separately heating each member or through the use ofmulti-conductor electrodes that heat select areas of the implant andheat transfer models of the implant to estimate the overall heatingprofile of each implant region. The controller using, its model,lookup-table or feedback control or direct measurement of the implanttemperature will allow the surgeon to plan the fixation strategy so asto optimize the biomechanical construct for each unique bone fixationcondition.

The subject invention controls maximum force and rate of implant changethrough fine control of the total heat energy applied to the implant.The system can also control the extent of opening in the fewmusculoskeletal indications where bone transport occurs and there islittle or no tissue force. Algorithms, numerical or graphical models,lookup table of settings or measurements of implant temperature andextent of shape change, combined with a current source will give thesurgeon the control necessary to utilize memory alloy implants to theirfull potential for musculoskeletal reconstruction.

For the first time the subject invention provides force and shape changerate control through controlling the implant temperature so that it iswithin the temperature range which the martensitic to austenitictransformation occurs for a given implant composition. Throughcontrolling the extent of the material's transformation the subjectinvention is able to control the implants applied force, rate ofclosure, and extent of closure together or separately while maintainingthe surface temperature at a level that will not cause thermal necrosisof the surrounding tissue.

This novel approach to controlling the implant for the first time allowsthe surgeon to program a implant to provide a range of forces or closingrates so as to meet the clinical requirements. This allows the surgeonto adjust the implant once placed in the body to get the requiredmusculoskeletal effect.

Accordingly, several objects and advantages of this invention are hereindescribed: 1) controls the rate of shape change of memory alloy devices,2) controls the compression or distraction forces between bones, 3)provides control on the relative positions of bones and angular changesin multiple bone structures, 4) controls implant heating profilesthrough a lookup table which contains at a minimum force, rate andimplant model information and 5) controls implant heating profilesthrough measurement of the temperature of the staple and using thisinformation in a feedback control loop set to a defined rate and force.These objects and advantages are achieved in addition to the features ofother inventions that limit the temperature of the implant to a levelbelow that which will cause tissue death. Further objects and advantagesof the subject invention will become apparent from a consideration ofthe drawings and ensuing description.

SUMMARY OF THE INVENTION

The subject invention is a system to control the surgical heating of animplant formed from memory metal so as to control its rate of shapechange and the forces it exerts on surrounding bone or in bone transportthe extent of bone movement. These variables will be controlled whilekeeping the implant's surface temperature clinically below the point ofthermal necrosis of tissue. The invention consists of an electrode andelectrical console that contacts the implant at a plurality of locationsand delivers to each location a selected amount of heat energy over aselected period of time. This allows the surgeon to slowly close animplant or an implant element and optimize bone fixation and position.

The subject invention accounts for the variables associated with theshape changing heat response of the memory metal implant to heat energysuch as resistive heat (electrical current flow), conductive heat(contact heating element), inductive heat (such as electromagnetic ormicrowave). The primary variables which cause a certain implanttemperature and shape change response are: 1) magnitude of energy flow,2) duration energy of flow, 3) mass of the implant, 4) shape of theimplant, 5) initial state of the implant, 6) impedance of the implant,7) thermal stability of the implant and 8) environment of the implant.The subject invention through a mathematical model, family of lookuptables or direct measurement of the temperature uses a current sourceand user defined inputs to controls the implant kinematics of shapechange.

BRIEF DESCRIPTION OF THE DRAWINGS

Further objects and advantages of this invention will become apparentfrom consideration of the drawings and ensuing description of thepreferred embodiment.

FIG. 1 Memory metal implant temperature versus heating duration curve

FIG. 2 Implant tissue force versus implant temperature curve

FIG. 3 Implant tissue force versus shape change period curve

FIG. 4 System diagram based on lookup table

FIG. 5 Lookup table example

FIG. 6 System diagram based on models, algorithms and lookup tables

FIG. 7 Multi-element implant example

FIG. 8 Multi-element implant electrode example

FIG. 9 Electrode handle and system circuit diagram for simple feedbackand control system example

LIST OF REFERENCE NUMERALS

100 Two second heating curve for martensitic to austinetic phasetransformation

110 Ten second heating curve for martensitic to austinetic phasetransformation

200 Austinetic transformation start temperature and inflection in forcecurve

210 Austinetic transformation end temperature

220 Minimum tissue force seen at body temperature

230 Maximum tissue force seen at body temperature

300 Force versus time curve while heating

400 Power supply with isolation transformer

410 AC to DC power converter

420 User set timer circuit

430 User set power circuit

440 Current delivery electrode

450 Circuit board

460 Electrode conductor to implant

470 Heating control button

475 Implant-system continuity light

480 Implant-system heat energy light

485 Lookup table

490 Front control panel with time, power, on-off switch, and indicatorlights

495 Audible operational indicator

500 Implant force and rate control lookup table

510 Heating duration data

520 Force level data

530 Power level within cells of the lookup table

600 Power supply with isolation transformer

610 AC to DC power converter

620 Timer circuit

630 Front panel control with keyboard and monitor

640 Current delivery electrode

650 Microprocessor running model, algorithm or sorting the lookup table

655 Circuit board

660 Audible operational indicator

670 Power circuit

680 Electrode handle

685 Electrode start button

690 Implant-system continuity light and heat energy light combined

695 Electrode conductor to implant

700 Spinal plate

705 First length or angle shape-changing member

710 Second length or angle shape-changing member

720 First bone anchoring shape-changing member

730 Second bone anchoring shape-changing member

740 Third bone anchoring shape-changing member

750 Fourth bone anchoring shape-changing member

760 First electrode contact point for member 720

765 Second electrode contact point for member 720

770 First electrode contact point for member 710

775 Second electrode contact point for member 710

780 First electrode contact point for member 750

785 Second electrode contact point for member 750

800 First electrode element for FIG. 7 member 720

810 First electrode element for FIG. 7 member 730

820 First electrode element for FIG. 7 member 740

830 First electrode element for FIG. 7 member 750

840 Second electrode element for FIG. 7 member 740

850 Second electrode element for FIG. 7 member 730

860 Second electrode element for FIG. 7 member 720

870 Second electrode element for FIG. 7 member 750

880 Multi-electrode handle

890 Electrode conductor bundle

895 Individual electrode conductors

900 Power supply

910 User operated switch

920 Temperature cutout switch

930 Thermocouple temperature sensing transducer

940 Implant heating electrode with thermocouple

950 Implant heating electrode

960 Thermocouple leads to temperature cutout switch

DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred embodiment of the invention consists of an electroniccontrol console that operates using a lookup table, algorithm ormathematical model to control the temperature of a memory ahoy implantin such a manner so as to control the extent of its transformation froma martensitic to austenitic microstructure. The rate of heat applicationcontrols the rate of implant shape change (FIG. 1). Rapid heating curves(100) and slow controlled heating curves (110) both can providesufficient heat energy to fully convert the material's phase and shape.The magnitude of heat transferred controls the state-of-the-metal'sphase change thus the force exerted on surrounding tissue (FIG. 2).Force and temperature relationships exist for each shape-changingelement of an implant. In an implant restrained in bone the forceexerted by the implant increases with temperature. The tissue forcebegins at or near the austinetic start temperature [A_(s)] (200) andincreases until the austinetic finish temperature [A_(f)] (210) as shownin the implant temperature versus force curve (FIG. 2). The force can becontrolled from a minimum value of the force at body temperature[F_(min)] (220) before heating and at body temperature after heating[F_(max)] (230). With these relationships for a given implantshape-changing member and power setting, a force versus time curve willexist (FIG. 3). Consequently, a force versus power curve will exist forany shape-changing member at a constant heating period.

The control of the heat energy to the implant is implemented using adevice: having a power supply with electrical patient isolationtransformer (400), rectifying circuit to convert alternating to directcurrent (410), user controllable timing circuit (420), user controllablepower circuit (430), a user operated current delivery electrode (440),circuit board (450), bipolar current delivery electrode (460), heatingcontrol button (470), implant-system continuity light (475),implant-system heat energy light (480), lookup table (485), frontcontrol panel with time, power, on-off switch, and indicator lights(490), and an audible operational indicator (495) (FIG. 4). The lookuptable (485) could be in the form of an alphanumeric table, mathematicalmodel, or algorithm.

The primary data from the surgeon consists of the 1) the implantselected, 2) the period of which the implant should change shape, and 3)the percentage of the total force available in this style of implant tobe applied. to the surrounding tissue. Each implant has a separatelookup table (500) (FIG. 5). This table can be within an operator'smanual or embedded in the instrument.

The rate data, time for the implant to dose, is used as row (510) orcolumn (520) headings and the force is the alternate column heading..The voltage setting to achieve the desired implant result is read fromwithin the cells (530) of the table.

Operation of the Invention

In the operation of , the preferred embodiment, the surgeon will selectthe implant to be used; maximum level of force for the implant to .apply to the surrounding tissue, and the amount of time that the implantshould take to change shape. Other variables such as ambient temperatureof the implant, amount of bone movement expected, and .extent of shapechange when correcting bone angulation can also be inputted or listed inthe lookup table (485).

Once the instrument is connected to a source of electricity so as toenergize the power supply (400), the surgeon after review of the lookuptable (485) sets the controls on the front panel of the instrument(490). After placing the implant into bone the surgeon will bring theelectrodes (460) into contact with the implant. The electrodes (460)when touching the implant cause the continuity light (475) to illuminateto show the surgeon that the force and rate control system is in optimalcontact with the implant to deliver a user selected amount of heatenergy over a user specified period of time. The surgeon then appliesheat energy to the implant by actuating the heating control button(470). As the heating current flows the continuity light (475) turns offand the heat energy light (480) illuminates during the user-selectedperiod for heat energy delivery. The front panel (490) time control knobsets the timing circuit (420) so as to control the rate of heat energydelivery and shape change period. Audible current flow indicator (495)assists the surgeon in the use of the system. The continuity light (475)and heat energy light (480) are located on the electrode handle (440) soas to be in dear view of the surgeon when working in the operative site.The circuit board (450) holds the electrical conductors, user controlledpower circuit (430) and the other components required to complete thesystem.

If, as in most cases in bone fixation, the bony elements do not move inresponse to the force applied by the implant the force magnitude appliedto tissue is as listed in the lookup table. If bone transport occursthen the force will be less than predicted. Under this condition thesurgeon can measure the amount of bone transport, input these data intothe lookup table and correct to obtain the actual applied force. Thisallows the surgeon to adjust the amount of displacement of bone as wellas force exerted on bone during the implantation procedure. Thisprovides fine control for the physician when stabilizing bone elementsor fragments.

Once the heating energy is delivered the energy light (480) turns offand the cycle is complete. The heating energy can be applied multipletimes to the implant. In the condition of osteoporosis the surgeon maysequentially increase the closing force of the implant through steppingthe closing force up at 10% to 20% increments until the surgeon receivesoperative clues that the maximum implant fixation force has been appliedwithout causing fracture of the osteoporotic bone. The subject inventionfor the first time gives full control to the physician to provide aplurality of implant force and closing rate characteristics to each ofplurality of implant designs.

Description of the First Alternate Embodiment

The first alternate embodiment of the invention is based on the sameprincipals of the preferred embodiment which controls the conversion ofthe martensitic to austenitic phase transition of the implant material.This first alternate embodiment integrates elements such as but notlimited to a lookup table, algorithms, heat transfer models of theimplants (640) and predictive graphics model of the shape change of theimplant and force applied to the tissue. These image and numeric dataare displayed on the front panel (630) monitor and controlled using thefront panel (630) keyboard to allow the surgeon to observe thetheoretical effects of heating to the implant and adjust the heat energyto the implant to get the desired clinical effect.

In this embodiment the power supply (600), AC to DC converter (610), andcircuit board (655) are configured to support a microprocessor (650) andcomputer memory which contains model data, model algorithms, and lookuptables (640) to allow the modeling of the effects of heat energyapplication to the implant. This intelligent system front panel (630)displays implant images, shape change data, and allows the modelpredictions of the rate and force of implant shape change to be comparedto the operative observations. The timer relay (620) and power circuit(670) in this embodiment are microprocessor (650) controlled. An audiblecurrent flow indicator (660) and combined continuity and heat energylight (690), in addition to the front panel (630) enhance surgeonfeedback to the operation of the subject invention in this embodiment.In this configuration the surgeon will program the force and rateprofile and then using the electrode handle (680) place the electrodes(695) on the point of the implant shown on the front panel (630) monitorto apply heat. Once the electrode (695) is in contact with the elementof the implant the electrode start button (685) is pushed, the frontpanel (630) monitor then displays the theoretical effect on the implantand directs the surgeon to the next element of the implant for heatenergy application.

Operation of the First Alternate Embodiment of the Invention

This first alternate embodiment allows the surgeon to predict thechanges to the implant and then observe the in vivo effects of theimplant on bony elements. These features increase the feedback to thesurgeon, compensate for and controls heating of multiple elements of animplant and enhances the degree of force and rate control of the implantfor the surgeon.

In use the surgeon will select and display the implant on the frontpanel (630) monitor. Then the desired rate and tissue force will beinput by the surgeon, using the front panel (630) for eachshape-changing element incorporated into the implant. Once set the frontpanel (630) monitor will guide the surgeon in using the electrode handle(680). The monitor will display the point of contact for the electrode(695) and instruct the surgeon to actuate the electrode start button(685). The monitor will then display the theoretical shape change andtissue force and guide the surgeon to place the electrode handle (680)on the next shape-changing member of the implant.

As shown in the spinal plate of FIG. 7 multiple shape changing membersmay need to be controlled. Members that lock into bone (720, 730, 740,and 750) or shrink to shorten the plant (705 and 710) can be selectivelycontrolled. In FIG. 7 member 720 can be affected by applying current tocircular points 760 and 765. Member 750 can be affected by applying heatenergy to points 780 and 785. And length shortening or angle changingmember 710 can be affected by applying heating energy across “x” markedpoints 770 and 775. A single bipolar electrode can be used at multiplelocations to close each shape-changing member one at a time.Alternatively a multi-electrode handle FIG. 8 can be placed on theimplant and the subject invention can then heat each implant shapechanging element (FIG. 7) in the selected sequence. In this mannerelectrode handle 880 can heat the shape-changing member 720 by havingthe electrode handle apply current to electrode points 830 and 870. Ormember 705 of FIG. 7 can be shortened or lengthened by applying heatenergy with the handle (880) and electrodes 840 and 850.

During the process the surgeon can adjust the force applied to thetissue, correct for bone transport that may make the force estimateinaccurate and then instruct the surgeon when the implant is in itsfinal configuration.

Description of the Second Alternate Embodiment

In the second alternate embodiment the implant-heating model and lookuptable are replaced with a measurement of the implant temperature. Thistemperature measurement is taken from the surface of the implant.Temperature measurement devices include but are not limited tothermocouples and thermal imaging. Other feedback mechanisms such asstrain gauges that measure the shape change of the implant can be usedfor feedback control. These measured data are input to the model or aswitch that stops current flow to the implant.

The system diagram of FIG. 6 now takes data from the heat sensingtransducers to input into the algorithm and implant models (640) so thatthe microprocessor (650) can correlate force and rate data withtemperature and accurately control the implant's shape change. Thismethod accounts for the environmental temperature issues associated witha cold operating room and a warm patient.

The FIG. 9 system diagram illustrates the temperature feedbackembodiment of the invention. Here the power supply (900) is connected tothe user-actuated electrode start button (910) which provides power to acurrent cutout switch (920) that receives input from the thermocouple(930), located on the electrode (940), through the thermocouple wires(960). The electrode (950) could hold an additional thermocouple forproduct redundancy and additional points of temperature measurement.

Operation of the Second Alternate Embodiment of the Invention

In this embodiment the amount of power set on the power supply (900)will control the rate of heating and the setting for the cutout switch(920) is related to the force. In the simple configuration of FIG. 9 alookup table is used to set the power level and the cutout parameters.

In use the surgeon selects these two parameters and the systemautomatically cuts out at the implant temperature corresponding to aspecific tissue force.

Conclusions, Ramifications and Scope of the Invention

The reader will see that the system and method described in thespecifications to control the rate, displacement and force of a shapechanging implant provides an important modality for the surgeon in thetreatment of skeletal injury and disease.

While the above description is specific this should not be construed aslimitations of the scope of the invention, but rather as an example of aplurality of possible embodiments which exhibit the characteristics ofcontrolling implants formed from shape memory material. Thus any systemthat individually or in concert controls the rate, displacement andforce exerted on tissue by a shape memory material implant is within thescope and spirit of this invention.

Accordingly, the scope of the invention should be determined not by theembodiments illustrated, but by the appended claims and their legalequivalents.

1-8. (canceled)
 9. A method comprising: (a) selecting an implantcomprising a memory material having a shape changing transitiontemperature range that has a shape that is designed to be controlledwith an energy delivery device, wherein (i) the memory materialcomprises martensitic phase memory material, (ii) the martensitic phasememory material is operable to transition to austenitic phase memorymaterial in the shape changing transition temperature range, and (iii)the step of selecting the implant is based, at least in part, on amaximum level of force the implant can apply when changed in shape; (b)implanting the implant during a surgical procedure; (c) utilizing theenergy delivery device to deliver an amount of energy to change theshape of the implant by transitioning at least some of the martensiticphase memory material to the austenitic phase memory material, wherein(i) the amount of energy delivered by the energy deliver device isselected to change the shape of the implant to a selected shape having aselected amount of force from a plurality of pre-selected increments ofenergy that correspond to a plurality of incremental shape changingforces that change the shape of the implant to a corresponding pluralityof incremental shape change positions, (ii) the corresponding pluralityof incremental shape change positions are positions of the implant thatsuccessively further change the shape of the implant, and (iii) theenergy delivery device is used to control (A) the rate of change in theshape of the implant and (B) the amount of energy delivered by theenergy delivery device to the implant.
 10. The method of claim 9,wherein the implant is selected from the group consisting of catheters,intrauterine contraceptive devices, gastrointestinal compression clip,blood vessel filter, coronary artery stent, skin staple, bone staple,and bone plate.
 11. The method of claim 9, wherein the implant comprisesa plurality of members, wherein the plurality of members are operable tobe controlled together by the energy delivery device.
 12. The method ofclaim 9, wherein the implant comprises a plurality of members, whereinthe plurality of members are operable to be controlled independently bythe energy delivery device.
 13. The method of claim 9, wherein thememory material has martensitic to austenitic finish transitiontemperatures sufficiently different so that the plurality of incrementalshape change positions can be implemented so as to achieve highresolution control of a characteristic selected from the groupconsisting of an implant force, rate of shape change, extent of shapechange.
 14. The method of claim 9, wherein the change in the shape ofthe implant is opening or closing of the implant.
 15. The method ofclaim 9, wherein the plurality of the pre-selected increments arepre-selected to control a pre-selected characteristic selected from thegroup consisting of (a) a corresponding plurality of incremental shapechange forces, (b) a corresponding plurality of periods of time ofincremental rate of shape change of the implant, (c) a correspondingplurality of extents of shape change of the implant, and (d)combinations thereof
 16. The method of claim 15, wherein thepre-selected characteristic comprises a combination of (a) thecorresponding plurality of the incremental shape change forces, (b) thecorresponding plurality of the periods of time of the incremental rateof shape change of the implant, and (c) the corresponding plurality ofthe extents of shape change of the implant.
 17. The method of claim 15,wherein the pre-selected characteristic comprises the correspondingplurality of the incremental shape change forces.
 18. The method ofclaim 17, wherein the corresponding plurality of the incremental shapechange forces are incrementally between 10% and 20% increments of themaximum level of force.
 19. The method of claim 17, wherein thecorresponding plurality of the incremental shape change forces are 10%,20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, and 100% of the maximum level offorce.
 20. The method of claim 17, wherein the pre-selectedcharacteristic further comprises the corresponding plurality of theperiods of time of the incremental rate of shape change of the implant.21. The method of claim 17, wherein the pre-selected characteristicfurther comprises the corresponding plurality of the extents of shapechange of the implant.
 22. The method of claim 15, wherein thepre-selected characteristic comprises the corresponding plurality of theperiods of time of the incremental rate of shape change of the implant.23. The method of claim 22, wherein the corresponding plurality of theperiods of time of the incremental rate of shape change of the implantare incrementally one second increments.
 24. The method of claim 22,wherein the pre-selected characteristic further comprises thecorresponding plurality of the extents of shape change of the implant.25. The method of claim 15, wherein the pre-selected characteristiccomprises the corresponding plurality of the extents of shape change ofthe implant.
 26. The method of claim 25, wherein the correspondingplurality of the extents of shape change of the implants areincrementally between 10% and 20% increments.
 27. The method of claim25, wherein the corresponding plurality of the extents of shape changeof the implant are 10%, 20%, 30%, 40%, 50%, 60%, 70%, 80%, 90%, and100%.
 28. The method of claim 9, wherein the implant is operable to bebeing controlled with the energy delivery device using a feedbackcontrol loop.
 29. The method of claim 28, wherein the feedback controlloop measures temperature of the implant.